(a) Field of the Invention
The present invention relates to an image-forming optical system. More specifically, the present invention relates to a miniature image-forming optical system for cameras using an image pickup device such as a charge coupled device (CCD).
(b) Description of the Related Art
Recently, the use of electronic still cameras and video cameras employing a CCD or a solid-state image pickup device has been expanding rapidly, and the needs for miniaturization and weight/cost savings are increasing. With the desire for greater compactness, miniature cameras such as mobile camera, web camera, or the like are increasingly used. Such miniaturization and weight/cost savings are also needed for the image-forming optical systems that are built into cameras.
A long retrofocal length must be taken into consideration in designing an optical system using a solid-state image pickup device. The optical system employs a crystal filter to prevent moiré caused by the periodical structure of the image pickup device, and a protective glass plate necessarily used due to the structure of the image pickup device. Therefore, the thickness and location of the crystal filter and the protective glass plate must be considered to secure a long retrofocal length in designing the optical systems. The optical system of which the image pickup device is sensitive to the light in the infrared (IR) region may have a problem of “saturation,” so there is a need for additionally using an infrared (IR) cut-off filter to reduce the sensitivity of the image pickup device to the IR region and to enhance transmissibility in the visible region. Thus such a long retrofocal length must be secured.
In addition, the optical system is also required to allow entrance of a large amount of light into lenses for the sake of compensation of a deterioration of the sensitivity caused by a high pixel value of the image pickup device, and to secure a large amount of peripheral light with respect to the center because of such a low sensitivity of the image pickup device.
The conventional image-forming optical systems using a solid-state image pickup device designed to satisfy these requirements are disclosed in Japanese Patent Application Laid-Open Nos. 2002-372666, 2002-296496, and 2003-75719.
These conventional optical systems are usually comprised of at least eight lenses in order of a first lens group having a positive refractive power, an iris diaphragm, and a second lens group having a negative refractive power. The use of such a large number of lenses makes it difficult to realize compactness of the optical system.